In recent years, it has been attempted vigorously to use a quantum confinement structure such as a quantum well or a quantum dot as a material for solar cells. This is motivated by that the band gap of the quantum confinement structure can be controlled by the size of confinement. The solar cell can most effectively utilize a light having an energy near the band gap of a light absorption layer. Therefore, if the band gap can be set to an energy near the peak of an intensity in solar spectra, a high efficiency solar cell can be attained.
Further, when quantum confinement effect is used, different band gaps can be attained depending on the confinement size even for an identical material. Therefore, it is expected, for example that a tandem solar cell comprising a plurality of sub cells having different band gaps can be manufactured simply by using the quantum confinement structure.